Coatings having low emissivity and low solar reflectance

ABSTRACT

The invention provides low solar reflectance, low-emissivity coatings. The invention provides a monolithic pane bearing a low solar reflectance, low-emissivity coating. Further, the invention provides an insulating glass unit bearing a low solar reflectance, low-emissivity coating. Finally, the invention provides methods of producing coated substrates by depositing low solar reflectance, low-emissivity coatings.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Patent Application No.60/411,031 filed on Sep. 16, 2002, and U.S. Patent Application No.60/376,826 filed on Apr. 29, 2002, the entire disclosure of each ofwhich is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention provides coatings for glass and other substrates.More particularly, this invention provides low-emissivity coatings thathave low solar reflectance. The invention also provides methods ofproducing coated substrates by depositing coatings of this nature, aswell as insulating glass units and monolithic panes bearing thesecoatings.

BACKGROUND OF THE INVENTION

Windows can reflect a surprising amount of solar radiation. In somecases, this reflected radiation can become problematic. A certain amountof energy is, of course, carried in the solar radiation reflected offthe exterior of a window. When this radiation falls on a nearby surface,the surface can be discolored. While this can occur even with a windowhaving clear uncoated glass, the problem can be more significant whenthe window bears a coating that is highly reflective of solar radiation.This problem can also be more significant if the panes of the window inquestion have become inwardly cupped. (The panes of an IG unit canbecome cupped, for example, during cold weather when gas in the interiorof the unit contracts.) The concave exterior pane of such a window wouldconcentrate its reflected radiation at a focal point exterior to thewindow. This focal point would tend to move as the sun moves across thesky, thus potentially leaving elongated paths of discoloration.

As noted above, solar reflection problems can be particularlysignificant for windows and other glazings (e.g., doors, skylights,etc.) that bear reflective coatings, such as low-emissivity coatings.Low-emissivity coatings are well known in the present art. Thesecoatings commonly include one or more reflective silver layers and twoor more transparent dielectric layers. The silver layers in thesecoatings are highly reflective of infrared radiation. Thus, theyfavorably reduce the transmission of radiant heat through the coating.However, these coatings also tend to have relatively high solarreflectance. For example, a window bearing a conventional low-emissivitycoating would typically have a solar reflectance of at least about30%–35%, while the solar reflectance of a window having clear uncoatedglass would typically be around 13%. Thus, from the perspective of solarreflection problems, conventional low-emissivity coatings are less thanideal. Accordingly, it would be desirable to provide a low-emissivitycoating that has low solar reflectance.

It would be particularly desirable to provide a low-emissivity coatingthat has low solar reflectance and also provides significant shadingproperties. As is well known, the solar heat gain coefficient (SHGC) ofa window is the fraction of incident solar radiation that is admittedthrough a window. There are a number of applications where low solarheat gain windows are of particular benefit. In warm climates, it isespecially desirable to have low solar heat gain windows. For example,solar heat gain coefficients of about 0.4 and below are generallyrecommended for buildings in the southern United States. Similarly, anywindows that are exposed to a lot of undesirable sun preferably have alow solar heat gain coefficient. For example, windows on the east orwest side of a building tend to get a lot of sun in the morning andafternoon. Likewise, sunrooms, solariums, and greenhouses typically geta great deal of sun. For applications like these, the solar heat gaincoefficient plays a vital role in maintaining a comfortable environmentwithin the building in question. Thus, it is beneficial to providewindows of this nature with coatings that establish a low solar heatgain coefficient (i.e., high shading ability coatings).

A tradeoff is sometimes made in high shading ability coatings wherebythe films selected to achieve a low SHGC have the effect of restrictingthe visible reflectance to a higher level than is desired. As aconsequence, windows bearing these coatings may have a somewhatmirror-like appearance. It would be desirable to provide a high shadingability coating that has sufficiently low visible reflectance to obviatethis mirror-like appearance problem.

In addition to having undesirably high visible reflectance, thetransmitted and reflected colors of conventional high shading abilitycoatings tend not to be ideal. For example, these coatings commonlyexhibit hues that are more red and/or yellow than is desired. To theextent a coating has a colored appearance, it is pleasing if the coatingexhibits a transmitted and/or reflected hue that is blue or blue-green.The chroma of these coatings tends also to be greater than is desired.In most cases, it is preferable to provide a coating that is as colorneutral (i.e., colorless) as possible. Thus, the reflected andtransmitted colors of conventional low solar heat gain coatings tend tobe less than ideal, both in terms of hue and chroma.

U.S. Patent Application No. 60/376,826 (Hoffman), the entire contents ofwhich are incorporated herein by reference, discloses advantageouslow-emissivity coatings that have low solar reflectance. These coatingsachieve an exceptional combination of properties, including particularlylow solar reflectance. In the '826 application, Hoffman describes fiveuniquely preferred low solar reflectance, low-emissivity film stacks.These film stacks are exceptionally well suited for a variety ofapplications. However, it would be desirable to improve these filmstacks in such a way that they impart greater insulating ability inwindows. For example, it would be desirable to achieve substantialdecreases in emissivity and U Value. Unfortunately, the changes requiredto decrease emissivity and U Value would be expected to cause anattendant decrease in visible transmittance and/or an attendantworsening of reflected or transmitted color. As skilled artisans willappreciate, overcoming this problem is an exceedingly difficult task,particularly considering the presence of the high absorption primarylayer in these coatings, which renders coating design highlyunpredictable.

SUMMARY OF THE INVENTION

In certain embodiments, the present invention provides a pane bearing alow-emissivity coating. In these embodiments, the low-emissivity coatingcomprises an infrared-reflective layer, a high absorption primary layer,and a middle coat. The infrared-reflective layer comprises material thatis highly reflective of infrared radiation. The infrared-reflectivelayer has a thickness of at least about 175 Å. The high absorptionprimary layer comprises material that is highly absorptive of solarradiation. The high absorption primary layer has a thickness of at leastabout 100 Å. The middle coat comprises at least one transparentdielectric film and is positioned between the infrared-reflective layerand the high absorption primary layer.

In certain embodiments, the invention provides a pane bearing alow-emissivity coating. In these embodiments, the low-emissivity coatingcomprises the following sequence of films (i.e., not necessarily in acontiguous sequence): an inner coat comprising at least one transparentdielectric film and having an optical thickness of between about 216 Åand about 312 Å; a high absorption primary layer comprising materialthat is highly absorptive of solar radiation and having a thickness ofleast about 100 Å; a middle coat comprising at least one transparentdielectric film and having an optical thickness of between about 600 Åand about 872 Å; an infrared-reflective layer comprising material thatis highly reflective of infrared radiation and having a thickness of atleast about 175 Å; a high absorption blocker layer comprising materialthat is highly absorptive of solar radiation and having a thickness ofat least about 45 Å; and an outer coat comprising at least onetransparent dielectric film and having an optical thickness of betweenabout 410 Å and about 582 Å.

In certain embodiments, the invention provides a method of producingcoated substrates. The method comprises providing a pane havinggenerally-opposed first and second major surfaces. Upon one of the majorsurfaces, there is deposited a low-emissivity coating comprising aninfrared-reflective layer, a high absorption primary layer, and a middlecoat. The infrared-reflective layer comprises material that is highlyreflective of infrared radiation. The infrared-reflective layer has athickness of at least about 175 Å. The high absorption primary layercomprises material that is highly absorptive of solar radiation. Thehigh absorption primary layer has a thickness of at least about 100 Å.The middle coat comprises at least one transparent dielectric film. Themiddle coat is positioned between the infrared-reflective layer and thehigh absorption primary layer. In some cases, the method comprisesdepositing the infrared-reflective layer as a silver-containing film.The method optionally comprises depositing the infrared-reflective layerat a thickness of between about 182 Å and about 274 Å. In some cases,the method comprises depositing the high absorption primary layer as ametallic film. The method optionally comprises depositing the highabsorption primary layer as a titanium and/or niobium containing film.In some cases, the method comprises depositing the high absorptionprimary layer as a highly absorptive dielectric film. The method canoptionally comprise depositing the high absorption primary layer at athickness of between about 104 Å and about 151 Å. In some cases, themethod comprises depositing the middle coat at an optical thickness ofbetween about 600 Å and about 872 Å. In some such cases, the methodcomprises depositing each film of the middle coat as a film having arefractive index of between about 1.7 and about 2.4. In some cases, themethod comprises depositing the infrared-reflective layer further fromthe substrate than the high absorption primary layer. In some suchcases, the method further comprises depositing a high absorption blockerlayer over the infrared-reflective layer, the high absorption blockerlayer comprising material that is highly absorptive of solar radiationand having a thickness of at least about 45 Å. Optionally, the methodcan comprises depositing the high absorption blocker layer directly overthe infrared-reflective layer. In some cases, the method comprisesdepositing the high absorption blocker layer as a metallic film. Themethod can optionally comprise depositing the high absorption blockerlayer as a titanium and/or niobium containing film. The method canoptionally comprise depositing the high absorption blocker layer at athickness of between about 46 Å and about 78 Å. In some cases, themethod further comprises depositing an inner coat between the substrateand the high absorption primary layer, the inner coat comprising atleast one transparent dielectric film. The method can optionallycomprise depositing the inner coat at an optical thickness of betweenabout 216 Å and about 312 Å. For example, the method can comprisedepositing each film of the inner coat as a film having a refractiveindex of between about 1.7 and about 2.4. In some cases, the methodfurther comprises depositing an outer coat further from the substratethan the infrared-reflective layer, the outer coat comprising at leastone transparent dielectric film. In some such cases, the methodcomprises depositing the outer coat at an optical thickness of betweenabout 410 Å and about 582 Å. For example, the method can optionallycomprise depositing each film of the outer coat as a film having arefractive index of between about 1.7 and about 2.4.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional perspective view of an insulating glass unitin accordance with certain embodiments of the present invention;

FIG. 2 is a schematic cross-sectional view of a low solar reflectancecoating in accordance with certain embodiments the invention;

FIG. 3 is a schematic side view of a sputtering chamber that has utilityin certain methods of the invention;

FIG. 4 is a schematic cross-sectional side view of a glazing carrying alow solar reflectance coating in accordance with certain embodiments ofthe invention;

FIG. 4A is a detailed cross-sectional side view of region 4A of the lowsolar reflectance coating carried by the glazing of FIG. 4;

FIG. 5 is a perspective view of a glazing that carries a low solarreflectance coating and has been mounted in the outer wall of a buildingin accordance with certain embodiments the invention;

FIG. 6 is a graph of the glass-side solar reflectance of a monolithicpane carrying a low solar reflectance coating in accordance with certainembodiments of the invention;

FIG. 7 is a graph of the transmitted color of an insulating glass unitcarrying a low solar reflectance coating in accordance with certainembodiments of the invention;

FIG. 8 is a graph of the exterior reflected color of an insulating glassunit carrying a low solar reflectance coating in accordance with certainembodiments of the invention; and

FIG. 9 is a graph of the solar transmittance of a monolithic panecarrying a low solar reflectance coating in accordance with certainembodiments of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The following detailed description is to be read with reference to thedrawings, in which like elements in different drawings have likereference numerals. The drawings, which are not necessarily to scale,depict selected embodiments and are not intended to limit the scope ofthe invention. Skilled artisans will recognize that the examplesprovided herein have many useful alternatives that fall within the scopeof the invention.

A variety of substrates are suitable for use in the present invention.In most cases, the substrate 10 is a sheet of transparent material(i.e., a transparent sheet). However, the substrate 10 is not requiredto be transparent. For most applications, the substrate will comprise atransparent or translucent material, such as glass or clear plastic. Inmany cases, the substrate 10 will be a glass pane. A variety of knownglass types can be used, and soda-lime glass is expected to bepreferred.

Tinted glass can optionally be used in certain embodiments of theinvention. Many suitable types of tinted glass are available from wellknown glass suppliers. Thus, a low solar reflectance coating of theinvention can be applied to a pane of tinted glass, if so desired. Insome cases, there is provided a multiple pane insulating glass unit (or“IG unit”) wherein the low solar reflectance coating is applied to apane of tinted glass, and this coated pane is incorporated (e.g., as anoutboard pane) into an IG unit that also includes at least one pane(e.g., an inboard pane) of clear glass. While embodiments of this natureare contemplated, the present low solar reflectance coating 40 isparticularly advantageous when used simply with clear glass.

In certain embodiments, the invention provides an IG unit that isprovided with at least one low solar reflectance coating. IG units arewell known in the present art. FIG. 1 depicts one example of an IG unit8 that can be provided in accordance with the invention. The invention,however, is not limited to practice with any particular type of IG unit.To the contrary, all aspects of the invention can be practiced with IGunits of any type (e.g., all-glass units, vacuum units, etc.). Thus, theillustrated IG unit type is not to be construed as limiting to theinvention. Further, while the term insulating “glass” unit is usedthroughout the present disclosure, it is to be understood that the panesneed not be formed of glass.

The IG unit 8 shown in FIG. 1 includes a first pane 10 and a second pane10′, together forming a pair of spaced-apart panes. The panes 10, 10′bound a between-pane space 115 therebetween and an exterior space 250thereabout. The panes have confronting inner surfaces 14, 14′ orientedtoward the between-pane space 115 and opposed outer surfaces 12, 12′oriented away from the between-pane space 115. In the embodiment of FIG.1, the panes 10, 10′ are held in a spaced-apart configuration (e.g., ina substantially parallel spaced-apart relationship) by a spacer 101. Thespacer 101 joins the peripheral inner surfaces of the panes. Thus, thespacer 101 and the confronting inner surfaces 14, 14′ of the panes 10,10′ together define the between-pane space 115. Useful IG units,components thereof, and methods of manufacturing and using IG units aredetailed in U.S. patent application Ser. No. 10/076,211, the entireteachings of which are incorporated herein by reference.

In the embodiment of FIG. 1, the illustrated IG unit 8 bears only onecoating 40. However, other coatings can be provided on one or more ofthe other major surfaces 12, 12′, 14′ of the IG unit 8, if so desired.For example, it may be desirable to provide a variety of differentcoatings on one or both outer surfaces 12, 12′ of the IG unit. Incertain embodiments, a hydrophilic coating (not shown) is provided onone or both outer surfaces 12, 12′. In one embodiment, the #1 surface ofan IG unit bears the hydrophilic coating, while the #2 surface bears thelow solar reflectance coating 40. Useful hydrophilic coatings aredisclosed in U.S. patent application Ser. Nos. 09/868,542, 09/572,766,and 09/599,301, the entire teachings of each of which are incorporatedherein by reference. In another embodiment, the #1 surface bears thehydrophobic coating, while the #2 surface bears the low solarreflectance coating 40. Useful hydrophobic coatings are disclosed inU.S. Pat. No. 5,424,130 (Nakanishi et al), the entire teachings of whichare incorporated herein by reference.

Further, certain embodiments provide an IG unit 8 wherein aphotocatalytic coating (not shown) is provided on one or both outersurfaces 12, 12′ of the IG unit 8. In one embodiment, the #1 surfacebears the photocatalytic coating, and the #2 surface bears the low solarreflectance coating 40. Useful photocatalytic coatings are described inU.S. Pat. No. 5,874,701 (Watanabe et al), U.S. Pat. No. 5,853,866(Watanabe et al), U.S. Pat. No. 5,961,843 (Hayakawa et al.), U.S. Pat.No. 6,139,803 (Watanabe et al), U.S. Pat. No. 6,191,062 (Hayakawa etal.), U.S. Pat. No. 5,939,194 (Hashimoto et al.), U.S. Pat. No.6,013,372 (Hayakawa et al.), U.S. Pat. No. 6,090,489 (Hayakawa et al.),U.S. Pat. No. 6,210,779 (Watanabe et al), U.S. Pat. No. 6,165,256(Hayakawa et al.), and U.S. Pat. No. 5,616,532 (Heller et al.), theentire teachings of each of which are incorporated herein by reference.

The improved low solar reflectance, low-emissivity coating 40 ispreferably carried on the “second” surface of an IG unit. This isperhaps best appreciated with reference to FIGS. 4 and 5, wherein thereis illustrated an IG unit 8 mounted upon a frame 90 in an exterior wall98 of a building 99. In such embodiments, the “first” (or “#1”) surfaceis that which faces (i.e., is exposed to, and communicates with) theoutdoor environment. Accordingly, it is the #1 surface that radiation SRfrom the sun 77 first strikes. In FIGS. 4 and 5, the outer surface 12 ofthe first pane 10 is the so-called first surface. Moving from the #1surface toward the interior side 33′, the next surface is the “second”(or “#2”) surface. As seen in FIG. 4, the inner surface 14 of the firstpane 10 is the so-called second surface. Moving further toward theinterior side 33′, the next surface is the “third” (or “#3”) surface,followed by the “fourth” (or “#4”) surface. In FIG. 4, the inner surface14′ of the second pane 10′ is the so-called third surface, and the outersurface 12′ of the second pane 10′ is the so-called fourth surface.

Thus, certain preferred embodiments of the invention provide an IG unitwherein an inner surface bears the low solar reflectance, low-emissivitycoating 40. The coating 40 includes an infrared-reflective layer 150 anda high absorption primary layer 80. The high absorption primary layer 80comprises titanium, niobium, or another material that is highlyabsorptive of solar radiation (e.g., a highly absorptive dielectric,such as titanium nitride). The high absorption primary layer 80desirably has a thickness of at least about 100 Å, preferably betweenabout 104 Å and about 151 Å, and more preferably between about 110 Å andabout 144 Å. The infrared-reflective layer 150 comprises silver oranother electrically-conductive material (e.g., metal), such as gold,copper, or the like. The infrared-reflective layer 150 desirably has athickness of at least about 175 Å, particularly preferably between about182 Å and about 274 Å, and more preferably between about 193 Å and about262 Å. The high absorption primary layer 80 is preferably positionedfurther to the exterior 77′ than the infrared-reflective layer 150, asis perhaps best appreciated with reference to FIGS. 4 and 4A.Preferably, the high absorption primary layer 80 is separated from theinfrared-reflective layer by a middle coat 90 comprising at least onetransparent dielectric film. Thus, certain embodiments provide a lowsolar reflectance, low-emissivity coating comprising: a high absorptionprimary layer (e.g., of the thickness and composition described in thisparagraph); an infrared-reflective layer (e.g., of the thickness andcomposition described in this paragraph); and a middle coat 90comprising at least transparent dielectric layer positioned between thehigh absorption primary layer and the infrared-reflective layer. Inthese embodiments, the coating 40 can optionally include inner 30 andouter 120 coats each comprising at least one transparent dielectricfilm.

In embodiments where the low solar reflectance coating 40 is carried onthe #2 surface of the IG unit 8, the high absorption primary layer 80 ispositioned closer to the pane 10 than the infrared-reflective layer 150.In some embodiments of this nature, the low solar reflectance coating 40comprises, in sequence from the substrate 10 outwardly (i.e., notnecessarily in a contiguous sequence): an inner coat 30 comprising atleast one transparent dielectric film (preferably having a thickness ofbetween about 108 Å and about 156 Å, more preferably between about 115 Åand about 150 Å, and perhaps optimally between about 128 Å and about 136Å); a high absorption primary layer 80 (e.g., comprising titanium,niobium, titanium nitride, or another highly absorptive material,preferably having a thickness of at least about 100 Å, more preferablybetween about 104 Å and about 151 Å, and perhaps optimally between about110 Å and about 144 Å); a middle coat 90 comprising at least onetransparent dielectric film (preferably having a thickness of betweenabout 300 Å and about 435 Å, more preferably between about 317 Å andabout 416 Å, and perhaps optimally between about 353 Å and about 378 Å);an infrared-reflective layer 150 (e.g., formed of silver or anotherelectrically-conductive material, preferably having a thickness of atleast about 175 Å, more preferably between about 182 Å and about 274 Å,and perhaps optimally between about 193 Å and about 262 Å); a highabsorption blocker layer 180 (e.g., comprising titanium, niobium, oranother highly absorptive material, preferably having a thickness of atleast about 45 Å, more preferably between about 46 Å and about 78 Å, andperhaps optimally between about 48 Å and about 75 Å); and an outer coat120 comprising at least one transparent dielectric film (preferablyhaving a thickness of between about 205 Å and about 291 Å, morepreferably between about 217 Å and about 278 Å, and perhaps optimallybetween about 242 Å and about 253 Å).

The present low solar reflectance, low-emissivity coating 40 has anumber of beneficial properties. The ensuing discussion reports severalof these properties. In some cases, these properties are reported in thecontext of a single pane bearing the present coating on one surface. Inother cases, these properties are reported in the context of an IG unithaving the present coating 40 on its #2 surface. In such cases, thereported properties have been determined for an IG unit wherein bothpanes are 3 mm soda-lime float glass, and wherein the IG unit has a ½inch between-pane space filled with an insulative gas mix of 90% argonand 10% air. Of course, these specifics are by no means limiting to theinvention. Absent an express statement to the contrary, the presentdiscussion reports determinations made using Window 4.1 under standardASHRAE conditions.

An IG unit bearing a conventional double silver low-emissivity coatingwould typically have an exterior (i.e., off the glass side of theoutboard pane) solar reflectance R_(s) of at least about 30%–35%. Giventhe solar reflection problems discussed above, it would be desirable toprovide a low-emissivity coating that offers lower solar reflection. Thepresent IG unit 8 achieves an exterior solar reflectance R_(s) of lessthan about 30%. In fact, the present IG unit 8 achieves an exteriorsolar reflectance R_(s) of less than about 20%. While the precise levelof solar reflection can be selected and varied in accordance with theteachings of this disclosure, certain preferred embodiments (e.g., wherethe coating 40 is one of the three uniquely preferred film stacksdetailed below) provide an IG unit 8 having an exterior solarreflectance R_(s) of about 16%.

The term “solar reflectance” is well known in the present art. This termis used herein in accordance with its well-known meaning to refer to thepercentage of incident solar radiation SR that is reflected off theglass side of a monolithic pane (which bears the coating 40 on theopposite film side) or off the exterior of the present IG unit 8.Skilled artisans will appreciate that the solar reflectance off theglass side of the monolithic pane includes not only solar radiationreflected at the surface 12, but also solar radiation reflected at thesurface 14. Likewise, the solar reflectance off the exterior side of theIG unit 8 (measured from the exterior 77 of the unit 8) includes notonly solar radiation reflected at the surface 12, but also solarradiation reflected at surfaces 14, 14′, and 12′. The reported solarreflectance is measured off a central portion of the glass side of themonolithic pane or off a central portion of the glass side of theoutboard pane 10 of the present IG unit 8, is indicated as R_(s) where sstands for solar. The solar reflectance can be determined as specifiedin “Standard Test Methods for Solar Energy Transmittance and Reflectance(Terrestrial) of Sheet Materials, ASTM”, the entire contents of whichare incorporated herein by reference.

FIG. 6 is a graph showing the glass-side reflectance of a monolithicpane bearing the present low solar reflectance coating (denoted by thesolid line) relative to the glass-side reflectance of a monolithic panebearing a double silver low-emissivity coating (denoted by the dashedline). The reflectance is reported in FIG. 6 for wavelengths betweenabout 300 nm and about 2,500 nm. This wavelength range is of interestbecause the solar radiation that reaches the earth is primarily in thisrange. In FIG. 6, it can be appreciated that the total solar reflectionof the present coating 40 is far less than that of the double-silvercoating. Thus, the present coating 40 offers exceptionally low solarreflection.

In addition to low solar reflectance, the present coating 40 hasexceptional shading ability. For example, the solar heat gaincoefficient (SHGC) of the present IG unit 8 is particularly low. As iswell known in the present art, the solar heat gain coefficient of awindow is the fraction of incident solar radiation that is admittedthrough the window. The term “solar heat gain coefficient” is usedherein in accordance with its well known meaning. Reference is made toNFRC 200-93 (1993), the entire teachings of which are incorporatedherein by reference.

As noted above, there are many applications where low solar heat gainwindows are of particular benefit. In warm climates, for example, it isdesirable to have low solar heat gain windows. Further, any windows thatare exposed to a lot of undesirable sun should have a low solar heatgain coefficient. For applications like these, the solar heat gaincoefficient plays a vital role in maintaining a comfortable environmentwithin a building. Thus, it is beneficial to provide windows of thisnature with coatings that establish a low solar heat gain coefficient.For example, a solar heat gain coefficient of about 0.4 or less iscommonly recommended for buildings in the southern United States andother warm climates.

The exceptional shading ability of the low solar reflectance coating 40is particularly beneficial for warm climate applications. For example,the present IG unit 8 has a solar heat gain coefficient of less thanabout 0.4. In fact, the IG unit 8 has a solar heat gain coefficient ofless than about 0.3, and preferably less than about 0.2. While theprecise level of shading ability can be selected and varied inaccordance with the teachings of this disclosure, certain preferredembodiments (e.g., where the coating 40 is one of the three uniquelypreferred film stacks detailed below) provide an IG unit 8 having asolar heat gain coefficient of about 0.16. Thus, the low solarreflectance coating 40 is particularly beneficial when high shadingability is desired.

A limitation of some high shading ability coatings is that they reflectmore visible light than is desired. As noted above, a tradeoff issometimes made in high shading ability coatings whereby the filmsselected to achieve a low SHGC have the effect of restricting thevisible reflectance to a level that is higher than ideal. As aconsequence, windows bearing these coatings may have a somewhatmirror-like appearance.

To the contrary, the present coating 40 has sufficiently low visiblereflectance to obviate this mirror-like appearance problem. For example,the exterior visible reflectance R_(v) of the present IG unit 8 is lessthan about 20%. In fact, the IG unit 8 achieves an exterior visiblereflectance R_(v) of less than about 18%. While the precise level ofvisible reflectance can be selected and varied in accordance with thepresent teachings, certain preferred embodiments (e.g., where thecoating 40 is one of the three uniquely preferred film stacks detailedbelow) achieve an IG unit 8 having an exterior visible reflectance R_(v)of about 11%. In contrast, the exterior visible reflectance of an IGunit having panes of clear uncoated glass would typically be about 15%.The extraordinarily low visible reflectance of the coating 40 isexceptional considering the great thickness of the infrared-reflectivelayer. This evidences the surprising results that are achieved by theparticular combination of films used in the present coating 40.

The term “visible reflectance” is well known in the present art. Thisterm is used herein in accordance with its well-known meaning to referto the percentage of all incident visible radiation that is reflectedoff the glass side of a monolithic pane (which bears the coating 40 onthe opposite film side) or off the exterior of the present IG unit 8.Skilled artisans will appreciate that the visible reflectance off theglass side of a monolithic pane includes not only visible radiationreflected at the surface 12, but also visible radiation reflected at thesurface 14. Likewise, the visible reflectance off the exterior side ofthe IG unit 8 (measured from the exterior 77 of the unit 8) includes notonly visible radiation reflected at the surface 12, but also visibleradiation reflected at surfaces 14, 14′, and 12′. The reported visiblereflectance is measured off a central portion of the glass side of themonolithic pane or off a central portion of the glass side of theoutboard pane 10 of the present IG unit 8, and is indicated as R_(v)where v stands for visible. Visible reflectance can be determined asspecified in the above-noted “Standard Test Methods for Solar EnergyTransmittance and Reflectance (Terrestrial) of Sheet Materials, ASTM”.

Reference is made once again to FIG. 6, wherein there is illustrated theglass-side reflectance of a monolithic pane bearing the present coating40 on one surface. Visible radiation comprises the wavelength range fromabout 380 nm to about 780 nm. As shown in FIG. 6, the glass-sidereflectance of a pane bearing the present coating 40 is about 10% over amajor portion (in fact, over substantially all) of the visiblewavelength range. Further, the reflectance is well below 20% (and, infact, does not exceed about 15%) over the entire visible range. Thus, itcan be appreciated that the present coating 40 offers exceptionally lowvisible reflectance.

FIG. 9 shows transmission properties of a monolithic pane bearing thepresent coating 40 on one surface. As can be appreciated, thetransmittance of the pane is highest across the visible range ofwavelengths. Peak transmittance occurs between the wavelengths of about400 nm and 450 nm and is about 18%–19%. Transmittance decreases atwavelengths outside the visible range. As will be appreciated by skilledartisans, these transmission properties are highly desirable for avariety of applications, such as high shading applications.

In addition to these beneficial properties, the present coating 40achieves color properties that are particularly pleasing. The followingdiscussion of transmitted and reflected color is reported using the wellknown color coordinates of “a” and “b”. In particular, these colorcoordinates are indicated herein using the subscript h (i.e., a_(h) andb_(h)) to represent conventional use of the well known Hunter Lab ColorSystem (Hunter methods/units, III. D65, 10 degree observer). The presentcolor properties can be determined as specified in ASTM D-2244-93,“Standard Test Method For Calculation Of Color Differences FromInstrumentally Measured Color Coordinates”, Sep. 15, 1993, as augmentedby ASTM E-308-85 Annual Book of ASTM Standards, Vol. 06.01 “StandardMethod For Computing The Colors Of Objects By Using The CIE System”, theentire contents of each of which are incorporated herein by reference.

The present IG unit 8 exhibits a transmitted color that is particularlypleasing. As noted above, it is commonly desirable for windows toexhibit hues of blue or blue-green. The transmitted hue of the presentIG unit 8 falls entirely within the blue-green range. In particular, theIG unit 8 exhibits a transmitted color characterized by an a_(h) colorcoordinate of between about −1.75 and about −4.5 and a b_(h) colorcoordinate of between about −2 and about −5. In certain preferredembodiments (e.g., where the low solar reflectance coating 40 is one ofthe three uniquely preferred film stacks detailed below), the IG unit 8exhibits a transmitted color characterized by an a_(h) color coordinateof between about −2.1 and about −4.2 and a b_(h) color coordinate ofbetween about −2.5 and about −4.5. This can be appreciated withreference to FIG. 7, wherein the transmitted color of such an IG unit 8is represented by the color box defined by the dashed lines. In thisfigure, it can be appreciated that the transmitted a_(h) and b_(h) colorvalues are both negative, such that the transmitted hue is in theblue-green range. Further, the magnitude of the negative a_(h) and b_(h)values is very low, indicating very low chroma/very good colorneutrality. Thus, when the present coating 40 is provided on an IG unit,the resulting unit 8 exhibits a particularly pleasing transmitted color.Accordingly, the present coating 40 is especially desirable forapplications where transmitted color is of particular interest, such asin sunrooms, solariums, greenhouses, and the like.

The present IG unit 8 also exhibits a very pleasing color in reflection.The reflected color reported herein is measured from the exterior 77′ ofthe IG unit 8. The present IG unit 8 is nearly colorless in reflection.In particular, the IG unit 8 exhibits a reflected color characterized byan a_(h) color coordinate of between about 1.4 and about −1.6 and ab_(h) color coordinate of between about 0.5 and about −2.5. In certainpreferred embodiments (e.g., where the coating 40 is one of the threeuniquely preferred film stacks detailed below), the IG unit 8 exhibits areflected color characterized by an a_(h) color coordinate of betweenabout 0.9 and about −1.2 and a b_(h) color coordinate of between about0.0 and about −2. This is shown in FIG. 8, wherein the reflected colorof such an IG unit 8 is represented by the color box defined by thedashed lines. In this figure, it can be appreciated that the chroma ofthe reflected color is exceptionally low, indicating that the coating 40is nearly colorless.

In addition to the beneficial properties discussed above, the present IGunit 8 has exceptional thermal insulating properties. As noted above,the low solar reflectance coating 40 includes at least oneinfrared-reflective film 150. This film 150 is highly reflective ofinfrared radiation (i.e., radiant heat). Since the infrared-reflectivefilm 150 is typically formed of silver or another electricallyconductive material, this film 150 contributes low emissivity to the lowsolar reflectance coating 40. For example, the emissivity of the presentcoating 40 is less than about 0.07. In fact, the emissivity of thiscoating 40 is less than about 0.05. While the precise level ofemissivity can be selected and varied in accordance with the presentteachings, a number of preferred coating embodiments (e.g., the threeuniquely preferred film stacks detailed below) provide an emissivity ofabout 0.044. In contrast, an uncoated pane of clear glass wouldtypically have an emissivity of about 0.84. Thus, the present coating 40achieves exceptionally low emissivity, and yet has excellent colorproperties and exceptionally low visible reflectance. This surprisingcombination of properties further evidences the extraordinary resultsthat are achieved by the particular combination of films used in thepresent coating 40.

The term “emissivity” is well known in the present art. This term isused herein in accordance with its well-known meaning to refer to theratio of radiation emitted by a surface to the radiation emitted by ablackbody at the same temperature. The present emissivity values can bedetermined as specified in “Standard Test Method For Emittance OfSpecular Surfaces Using Spectrometric Measurements” NFRC 301-93, theentire contents of which are incorporated herein by reference.

The “U Value” of the present IG unit 8 is also exceptionally low. As iswell known, the U Value of an IG unit is a measure of the thermalinsulating ability of the unit. The smaller the U value the better thethermal insulating ability of the unit. The U Value of the present IGunit 8 is less than about 0.4. In fact, the IG unit 8 has U Value ofless than about 0.3. While the precise level of U Value can be selectedand varied in accordance with the present teachings, certain preferredembodiments (e.g., where the coating 40 is one of the three uniquelypreferred film stacks detailed below) provide an IG unit 8 wherein the UValue is about 0.25. In comparison, the U Value of an IG unit havingpanes of uncoated glass would typically be about 0.46. Thus, the presentcoating 40 facilitates exceptionally low U Value.

The term U Value is well known in the present art. It is used herein inaccordance with its well-known meaning to express the amount of heatthat passes through one unit of area in one unit of time for each unitof temperature difference between a hot side of the IG unit 8 and a coldside of the IG unit 8. The U Value can be determined in accordance withthe standard specified for U_(winter) in NFRC 100-91 (1991), the entirecontents of which are incorporated herein by reference.

FIG. 2 depicts a preferred low solar reflectance coating 40 of theinvention. As can be appreciated, the illustrated coating 40 generallyincludes the following sequence of films, moving outwardly (i.e., awayfrom the substrate): a transparent dielectric inner coat 30; a highabsorption primary layer 80; a transparent dielectric middle coat 90; aninfrared-reflective layer 150; a high absorption blocker layer 180; anda transparent dielectric outer coat 120. The present disclosure teachesparticular combinations of thicknesses and materials for these films,which combinations achieve the exceptional properties described above.

As noted above, the present coating 40 includes an infrared-reflectivefilm 150. This infrared-reflective film 150 is preferably formed of anelectrically-conductive material (e.g., metal), such as silver, gold,copper, or the like. Alloys or mixtures of these metals can also beused. In most cases, it will be preferable to employ a silver orsilver-containing film (e.g., comprising a major weight percentage ofsilver). The term “silver-containing” is used herein to refer to anyfilm that includes at least some silver. For example, one may provide aninfrared-reflective film in the form of silver combined with a smallamount of gold (e.g., about 5% gold or less).

The infrared-reflective film 150 is highly reflective of infraredradiation. As a result, this film 150 substantially reduces thetransmission of radiant heat through the coating 40. Further, theelectrically-conductive material of this film 150 has low sheetresistance, and hence low emissivity. Thus, the infrared-reflective film150 contributes low emissivity to the coating 40. As noted above, theseproperties are desirable for coatings on windows and other glazings(e.g., doors, skylights, etc.). For example, during a cold winter it isdesirable to minimize the heat that escapes from a warm room through awindow to a cold outdoor environment. Likewise, during a warm summer itis desirable to minimize the heat that enters a cool room through awindow from a hot outdoor environment. Thus, the infrared-reflectivefilm 150 is advantageous in that it helps reduce the amount of heat thatpasses through the coating 40.

The infrared-reflective film 150 is preferably provided at particularthicknesses. The thickness of this film 150 is desirably at least about175 Å, preferably between about 182 Å and about 274 Å, more preferablybetween about 193 Å and about 262 Å, and perhaps optimally between about215 Å and about 238 Å. Forming the infrared-reflective layer 150,especially of silver or a silver-containing film, at these thicknessesis particularly preferred.

The low solar reflectance coating 40 preferably includes a highabsorption primary layer 80. The high absorption primary layer 80 ispreferably formed of particular materials. For example, this primarylayer 80 preferably comprises titanium, niobium, or another materialthat is highly absorptive of solar radiation (e.g., a highly absorptivedielectric material, such as titanium nitride). The high absorptionprimary layer 80 absorbs a substantial portion of incident solarradiation. In certain preferred embodiments, the primary layer 80comprises metallic titanium, metallic niobium, or another metallicmaterial that is highly absorptive of solar radiation. Thus, the layer80 may consist, or consist essentially, of a highly absorptive metallicmaterial. In some cases, all but a portion of the high absorptionprimary layer 80 is metallic. In such cases, the outer portion (i.e.,the portion furthest from the substrate) of this layer 80 may beoxidized, nitrided, or otherwise reacted to some extent. This tends tooccur when the high absorption primary layer 80 is deposited as metallicfilm, and the deposition of a subsequent film is performed in a reactive(e.g., oxidizing and/or nitriding) atmosphere. In such cases, the outerface of the primary layer 80 is exposed to the reactive atmosphereduring an initial period of the subsequent film deposition, such thatthe outer portion 80′ of the primary layer 80 is oxidized, nitrided,and/or otherwise reacted. In these embodiments, it is desirable if nomore than a minor portion (e.g., less than 50% of the thickness) of theprimary layer 80 is a reaction product (e.g., an oxide, nitride, and/oroxynitrides), and a major portion (e.g., 50% or more of the thickness)thereof is metallic. Thus, it can be appreciated that certainembodiments involve a high absorption primary layer 80 that consists, orconsists essentially, of a highly absorptive metallic material andreaction products of such metallic material.

The high absorption primary layer 80 is preferably provided atparticular thicknesses. The thickness of the high absorption primarylayer 80 is desirably at least about 100 Å, preferably between about 104Å and about 151 Å, more preferable between about 110 Å and about 144 Å,and perhaps optimally between about 123 Å and about 131 Å. Forming thehigh absorption primary layer 80 at these thicknesses is particularlypreferred, especially when this layer 80 is formed of particularmaterials, as will now be described.

In certain particularly preferred embodiments, the high absorptionprimary layer 80 comprises titanium. In one embodiment, this layer 80 isa titanium-containing film having a thickness within at least one of theranges described in the preceding paragraph. The term“titanium-containing” is used herein to refer to any film that containsat least some titanium. Thus, absent an express statement to thecontrary, materials other than titanium may be present in such a film.In some cases, the high absorption primary layer 80 is atitanium-containing film that consists, or consists essentially, oftitanium. In other cases, this layer 80 is a titanium-containing filmhaving an outer portion 80′ that is a reaction product of titanium(e.g., titanium oxide, titanium nitride, and/or titanium oxynitride). Insuch cases, it will generally be preferred if a major inner portion(i.e., 50% or more) of the titanium-containing film is metallictitanium, while a minor outer portion (i.e., less than 50%) is atitanium reaction product. For example, the high absorption primarylayer 80 can be a titanium-containing film wherein metallic titaniumaccounts for at least about 62 Å, more preferably at least about 75 Å,and perhaps optimally at least about 80 Å of this layer 80 (e.g., whereat least the innermost 62 Å, 75 Å, or 80 Å is metallic titanium). Incertain embodiments, the high absorption primary layer is deposited as ametallic titanium film.

In certain embodiments, the high absorption primary layer 80 comprisesniobium. In one embodiment, this layer 80 is a niobium-containing filmhaving a thickness within at least one of the described ranges. The term“niobium-containing” is used herein to refer to any film that containsat least some niobium. Absent an express statement to the contrary,materials other than niobium may be present in such a film. In somecases, the high absorption primary layer 80 is a niobium-containing filmthat consists, or consists essentially, of niobium. In other cases, thislayer 80 is a niobium-containing film having an outer portion 80′ thatis a reaction product of niobium (e.g., niobium oxide, niobium nitride,and/or niobium oxynitride). In such cases, it will generally bepreferred if a major inner portion of the niobium-containing film ismetallic niobium, while a minor outer portion is a niobium reactionproduct. For example, the high absorption primary layer 80 can be aniobium-containing film wherein metallic niobium accounts for at leastabout 62 Å, more preferably at least about 75 Å, and perhaps optimallyat least about 80 Å of this layer 80 (e.g., where at least the innermost62 Å, 75 Å, or 80 Å is metallic niobium). In certain embodiments, thehigh absorption primary layer is deposited as a metallic niobium film.

In certain embodiments, the high absorption primary layer 80 comprisesboth niobium and titanium. In one embodiment, this layer 80 is aniobium-titanium-containing film having a thickness within at least oneof the described ranges. The term “niobium-titanium-containing” is usedherein to refer to any film that contains at least some niobium and atleast some titanium. Absent an express statement to the contrary,materials other than niobium and titanium may be present in such a film.Useful niobium-titanium films and methods for their deposition aredescribed in U.S. patent application Ser. No. 10/123,032, filed on Apr.11, 2002 and entitled “Thin Film Coating Having Niobium-Titanium Layer”,the entire contents of which are incorporated herein by reference. Insome cases, the high absorption primary layer 80 is aniobium-titanium-containing film that consists, or consists essentially,of niobium and titanium. In other cases, this layer 80 is aniobium-titanium-containing film having an outer portion 80′ that is areaction product of a niobium-titanium material. In such cases, it willgenerally be preferred if a major inner portion of thisniobium-titanium-containing film is metallic niobium-titanium (e.g., analloy of niobium and titanium), while a minor outer portion is aniobium-titanium reaction product. For example, the high absorptionprimary layer 80 can be a niobium-titanium-containing film whereinmetallic niobium-titanium accounts for at least about 62 Å, morepreferably at least about 75 Å, and perhaps optimally at least about 80Å of this layer 80 (e.g., where at least the innermost 62 Å, 75 Å or 80Å is metallic niobium-titanium). In certain embodiments, the highabsorption primary layer is deposited as a metallic niobium-titaniumfilm.

In certain embodiments, the high absorption primary layer 80 comprises adielectric film that is highly absorptive of solar radiation. In onesuch embodiment, the high absorption primary layer 80 comprises (e.g.,consists essentially of) titanium nitride. Of course, skilled artisansmay wish to select other known high absorption dielectric films.

With continued reference to the preferred embodiment of FIG. 2, it canbe appreciated that the coating 40 preferably includes a high absorptionblocker layer 180. This blocker layer 180 is preferably depositeddirectly over the infrared-reflective film 150. The preferred highabsorption blocker layer 180 serves a number of purposes. For example,this layer 180 protects the underlying infrared-reflective film 150during the deposition of subsequent films. This blocker layer 180preferably comprises a metal or metal alloy that reacts readily withoxygen, nitrogen, or other reactive gas used in depositing subsequentfilms. This allows the blocker layer 180 to capture reactive gas thatwould otherwise reach and react with the infrared-reflective film 150.In addition, the high absorption blocker layer 180 provides theinfrared-reflective film 150 with exceptional protection againstchemical corrosion. This is believed to be a result of the relativelygreat thickness of the high absorption blocker layer 180, as compared toconventional blocker layers. The protective properties of the highabsorption blocker layer 180 are credited in part for the outstandingchemical durability that has been observed in the present coating 40.Further, the high absorption blocker layer 180 affords exceptionalcontrol over the transmitted color of the present coating 40. Asdescribed above, the transmitted color of the present coating 40 isexceptionally color neutral, and this is attributed in part to theparticular composition and thickness of the high absorption blockerlayer 180.

The high absorption blocker layer 180 is preferably provided atparticular thicknesses. The thickness of this layer 180 is desirably atleast about 45 Å, preferably between about 46 Å and about 78 Å, morepreferably between about 48 Å and about 75 Å, and perhaps optimallybetween about 54 Å and about 68 Å. Forming the high absorption blockerlayer 180 at these thicknesses is particularly preferred, especiallywhen this layer 180 is formed of particular materials, as will now bedescribed.

In a number of particularly preferred embodiments, the high absorptionblocker layer 180 comprises titanium. In certain embodiments, this layer180 is a titanium-containing film having a thickness within at least oneof the ranges described in the preceding paragraph. The high absorptionblocker layer 180 can be a titanium-containing film that consists, orconsists essentially, of titanium. Alternatively, this layer 180 can bea titanium-containing film having an outer portion that is a reactionproduct of titanium (e.g., titanium oxide, titanium nitride, and/ortitanium oxynitride). In such cases, it will generally be preferred if amajor inner portion of the titanium-containing film is metallictitanium, while a minor outer portion is a titanium reaction product.Thus, the high absorption blocker layer 180 can be a titanium-containingfilm wherein metallic titanium accounts for at least about 23 Å, morepreferably at least about 25 Å, and perhaps optimally at least about 27Å of this layer 180 (e.g., where at least the innermost 23 Å, 25 Å, or27 Å is metallic titanium).

In certain embodiments, the high absorption blocker layer 180 comprisesniobium. In some embodiments of this nature, the high absorption blockerlayer 180 is a niobium-containing film having a thickness within atleast one of the described ranges. The high absorption blocker layer 180can be a niobium-containing film that consists, or consists essentially,of niobium. Alternatively, this layer 180 can be a niobium-containingfilm having an outer portion that is a niobium reaction product. In suchcases, it will generally be preferred if a major inner portion of theniobium-containing film is metallic niobium, while a minor outer portionis a niobium reaction product. For example, the high absorption blockerlayer 180 can be a niobium-containing film wherein metallic niobiumaccounts for at least about 23 Å, more preferably at least about 25 Å,and perhaps optimally at least about 27 Å of this layer 180 (e.g., whereat least the innermost 23 Å, 25 Å, or 27 Å is metallic niobium).

In certain embodiments, the high absorption blocker layer 180 comprisesboth niobium and titanium. Useful niobium-titanium blocker layers aredescribed in the above-noted '032 patent application. In someembodiments of this nature, the high absorption blocker layer 180 is aniobium-titanium-containing film having a thickness within at least oneof the noted ranges. The high absorption blocker layer 180 can be aniobium-titanium-containing film that consists, or consists essentially,of a niobium-titanium material (e.g., alloys of niobium and titanium).Alternatively, the high absorption blocker layer 180 can be aniobium-titanium-containing film having an outer portion that is areaction product of niobium-titanium. In such cases, it will generallybe preferred if a major inner portion of the niobium-titanium-containingfilm is metallic niobium-titanium, while a minor outer portion ispresent in the form of a niobium-titanium reaction product. For example,the high absorption blocker layer 180 can be aniobium-titanium-containing film wherein metallic niobium-titaniumaccounts for at least about 23 Å, more preferably at least about 25 Å,and perhaps optimally at least about 27 Å of this layer 180 (e.g., whereat least the innermost 23 Å, 25 Å, or 27 Å is metallicniobium-titanium).

The low solar reflectance coating 40 is preferably provided with atransparent dielectric inner coat 30, a transparent dielectric middlecoat 90, and a transparent dielectric outer coat 120. The transparentdielectric films 30, 90, 120 are preferred to establish theexceptionally well-balanced properties of the present coating 40. Forexample, these preferred films reduce the visible reflectance of thecoating 40, control the color of the coating 40, and impart chemicaldurability in the coating 40. The preferred inner coat 30 is positionedbetween the substrate 10 and the high absorption primary layer 80, whilethe preferred outer coat 120 is positioned further from the substrate 10than the infrared-reflective film 150. In some cases, the preferredinner coat 30 is contiguous to the substrate 10. However, the inventionalso provides embodiments wherein a transparent base layer 20 (notshown) is positioned between the preferred inner coat 30 and thesubstrate 10. Useful transparent base layers 20 are described in U.S.patent application Ser. No. 10/087,662, the entire contents of which areincorporated herein by reference. In certain embodiments, the preferredouter coat 120 forms the outermost film region of the present coating40. Alternatively, a variety of overcoats can be positioned further fromthe substrate than the preferred outer coat, if so desired.

The preferred inner 30 and outer 120 coats each comprise at least onetransparent dielectric film. The term “transparent dielectric” is usedherein to refer to any non-metallic (i.e., neither a pure metal nor ametal alloy) compound that includes any one or more metals and issubstantially transparent when deposited as a thin film. For example,included in this definition would be any metal oxide, metal nitride,metal carbide, metal sulfide, metal boride, and any combination thereof(e.g., an oxynitride). Further, the term “metal” should be understood toinclude all metals and semi-metals (i.e., metalloids). In particular,useful metal oxides include oxides of zinc, tin, indium, bismuth,titanium, hafnium, zirconium, and alloys and mixtures thereof. Whilemetal oxides are advantageous due to their ease and low cost ofapplication, known metal nitrides (e.g., silicon nitride, titaniumnitride, etc.) can also be used advantageously. Skilled artisans will befamiliar with other useful transparent dielectric materials.

The preferred inner coat 30 is preferably provided at particularthicknesses. For example, the physical thickness of the inner coat 30 ispreferably between about 108 Åand about 156 Å, more preferably betweenabout 115 Å and about 150 Å, and perhaps optimally between about 128 Åand about 136 Å. In a first embodiment, the inner coat 30 is a singlezinc oxide film. In a second embodiment, the inner coat 30 is a singletitanium oxide film (e.g., titanium dioxide and/or substoichiometricTiO_(x), where x is less than 2). In a third embodiment, the inner coat30 is a single silicon nitride film. In a fourth embodiment, the innercoat 30 is a single tin oxide film. In each of these four embodiments,the thickness of the inner coat 30 is preferably within at least one ofthe ranges described in this paragraph.

In certain alternate embodiments (not shown), the inner coat 30comprises at least two films. The preferred inner coat 30 can be formedof essentially any desired number of films. However, the total opticalthickness of the inner coat 30 (whether it consists of one or multiplefilms) is preferably between about 216 Å and about 312 Å, morepreferably between about 230 Å and about 300 Å, and perhaps optimallybetween about 256 Å and about 272 Å. In certain embodiments, each filmof the inner coat 30 is a transparent dielectric film having arefractive index of between about 1.7 and about 2.4, and perhapsoptimally about 2.0.

The exceptional properties of the present coating 40 are due in part tothe thinness of the preferred inner coat 30. Excellent antireflectionand color is achieved by providing the preferred inner coat 30 at anoptical thickness of less than about 312 Å, more preferably less thanabout 300 Å, and perhaps optimally less than about 272 Å, whiledesirably having an optical thickness of at least about 216 Å.

The preferred outer coat 120 is also preferably provided at particularthicknesses. For example, the physical thickness of the outer coat 120is preferably between about 205 Å and about 291 Å, more preferablybetween about 217 Å and about 278 Å, and perhaps optimally between about242 Å and about 253 Å. In a first embodiment, the outer coat 120 is asingle zinc oxide film. In a second embodiment, the outer coat 120 is asingle titanium oxide film. In a third embodiment, the outer coat 120 isa single silicon nitride film. In a fourth embodiment, the outer coat120 is a single tin oxide film. In each of these four embodiments, thethickness of the outer coat 120 preferably is within at least one of theranges described in this paragraph.

In a number of preferred embodiments (not shown), the outer coat 120comprises at least two films. As with the inner coat 30, the preferredouter coat 120 can be formed of essentially any desired number of films.However, the total optical thickness of the outer coat 120 (whether itconsists of one or multiple films) is preferably between about 410 Å andabout 582 Å, more preferably between about 434 Å and about 556 Å, andperhaps optimally between about 484 Å and about 506 Å. In certainembodiments, each film of the outer coat 120 is a transparent dielectricfilm having a refractive index of between about 1.7 and about 2.4, andperhaps optimally about 2.0.

In certain preferred embodiments, the outer coat 120 comprises two outerfilms of different transparent dielectric materials. These films can beformed respectively of essentially any two transparent dielectricmaterials. In some cases, these films are contiguous to one another,although this is not required. In one embodiment, the outer coat 120comprises a first layer of zinc oxide and a second layer of siliconnitride positioned over (e.g., directly over) the zinc oxide layer.Alternatively, the first layer can be titanium oxide and the secondlayer can be silicon nitride. As still another alternative, the firstlayer can be tin oxide and the second layer can be silicon nitride. Asyet another alternative, the first layer can be zinc oxide and thesecond layer can be titanium oxide or tin oxide. The respectivethicknesses of these outer films can be selected and varied as desired.Preferably, the combined optical thickness of these two films is withinat least one of the ranges described in the preceding paragraph.

In embodiments where the outer coat 120 comprises multiple films, theoutermost of these films preferably comprises a chemically-durablematerial, such as silicon nitride. U.S. Pat. No. 5,834,103, the entirecontents of which are incorporated herein by reference, describessilicon nitride films that can be used advantageously as the outermostfilm in the present coating 40. In certain particularly preferredembodiments, the outermost film is silicon nitride deposited at athickness of between about 15 Å and about 46 Å, more preferably betweenabout 16 Å and about 44 Å, and perhaps optimally between about 18 Å andabout 40 Å.

A chemically-durable film of the nature (e.g., of the thickness andcomposition) just described can be deposited advantageously over (i.e.,further from the substrate than) an underlying, outer transparentdielectric film having a thickness of between about 177 Å and about 270Å, more preferably of between about 189 Å and about 259 Å, and perhapsoptimally between about 209 Å and about 235 Å. In certain embodiments,this underlying (e.g., directly underlying) transparent dielectric filmis formed of zinc oxide, titanium oxide, or tin oxide. In particular,the high sputtering rate of zinc oxide makes it a preferred material forthis underlying, outer transparent dielectric film.

The exceptional optical properties of the present coating 40 are due inpart to the thinness of the preferred outer coat 120. Excellentantireflection and color is achieved by providing the preferred outercoat 120 at an optical thickness of less than about 582 angstroms, morepreferably less than about 556 angstroms, and perhaps optimally lessthan about 506 angstroms, while desirably having an optical thickness ofat least about 410 angstroms.

The low solar reflectance coating 40 is preferably provided with atransparent dielectric middle coat 90 between the high absorptionprimary layer 80 and the infrared-reflective layer 150. The preferredmiddle coat 90 comprises at least one transparent dielectric film. Incertain preferred embodiments, the middle coat 90 is provided in theform of a single transparent dielectric film. This film can comprise anyof the transparent dielectric materials described above. In oneembodiment, the middle coat 90 is a single zinc oxide film.

The transparent dielectric middle coat 90 is preferably provided atparticular thicknesses. For example, the physical thickness of themiddle coat 90 is preferably between about 300 Å and about 435 Å, morepreferably between about 317 Å and about 416 Å, and perhaps optimallybetween about 353 Å and about 378 Å. Forming the transparent dielectricmiddle coat 90 at these thicknesses is particular preferred. Thethicknesses noted herein are physical thicknesses, unless specificallyidentified as being optical thicknesses.

In alternate embodiments (not shown), the middle coat 90 is provided inthe form of a plurality of transparent dielectric films. Whether themiddle coat 90 consists of one or multiple films, the overall opticalthickness of this coat 90 is preferably between about 600 Å and about872 Å, more preferably between about 636 Å and about 832 Å, and perhapsoptimally between about 706 Å and about 756 Å. In certain embodiments,each film in the middle coat 90 is a transparent dielectric film havinga refractive index of between about 1.7 and about 2.4, and perhapsoptimally about 2.0.

The exceptional optical properties of the present coating 40 are due inpart to the relative optical thicknesses of the preferred inner coat 30,the preferred middle coat 90, and the preferred outer coat 120. Forexample, in certain embodiments, there is provided a specific ratio ofthe optical thickness of the inner coat 30 relative to the opticalthickness of the middle coat 90. Additionally or alternatively, therecan be provided a specific ratio of the optical thickness of the outercoat 120 relative to the optical thickness of the middle coat 90.

In certain embodiments, the ratio of optical thickness of the inner coat30 to the optical thickness of the middle coat 90 is preferably betweenabout 0.28 and about 0.47, more preferably between about 0.34 and about0.39, and perhaps optimally about 0.35–0.36. Further, in certainembodiments, the ratio of the optical thickness of the outer coat 120 tothe optical thickness of the middle coat 90 is preferably between about0.52 and about 0.88, more preferably between about 0.64 and about 0.72,and perhaps optimally about 0.67–0.69. In certain preferred embodiments,the coating 40 has one of the foregoing ratios of inner coat/middle coatas well as one of the foregoing ratios of outer coat/middle coat.

Three uniquely preferred low solar reflectance film stack 40 embodimentswill now be detailed. Each of these film stacks is preferably utilizedas a second-surface coating. In particular, where one of these filmstacks is born on the #2 surface of an IG unit, the resulting unit 8achieves all of the beneficial properties noted above. While the presentdisclosure focuses somewhat on IG unit embodiments, it is to beunderstood that the invention extends to any substrate (e.g., amonolithic pane or a flexible sheet) carrying the present low solarreflectance, low-emissivity coating 40.

A first uniquely preferred low solar reflectance, low-emissivity filmstack has the following structure: (1) a zinc oxide layer depositeddirectly upon a glass sheet at a thickness of between about 110 Å andabout 150 Å, more preferably between about 117 Å and about 143 Å, andoptimally about 130 Å; (2) a titanium layer deposited directly upon thiszinc oxide layer at a thickness of between about 111 Å and about 151 Å,more preferably between about 118 Å and about 144 Å, and optimally about131 Å, wherein an outer portion of this titanium layer is oxidizedduring deposition of the overlying zinc oxide film in an oxidizingatmosphere; (3) a zinc oxide layer deposited directly upon this titaniumlayer at a thickness of between about 303 Å and about 411 Å, morepreferably between about 321 Å and about 393 Å, and optimally about 357Å; (4) a silver layer deposited directly upon this zinc oxide layer at athickness of between about 185 Å and about 251 Å, more preferablybetween about 196 Å and about 240 Å, and optimally about 218 Å; (5) atitanium layer deposited directly upon this silver layer at a thicknessof between about 46 Å and 62 Å, more preferably between about 49 Å and59 Å, and optimally about 54 Å, wherein an outer portion of thistitanium layer is oxidized during deposition of the overlying zinc oxidefilm in an oxidizing atmosphere; (6) a zinc oxide layer depositeddirectly upon this titanium layer at a thickness of between 181 Å andabout 245 Å, more preferably between about 191 Å and about 235 Å, andoptimally about 213 Å; and (7) a silicon nitride layer depositeddirectly upon this zinc oxide layer at a thickness of between about 34 Åand 46 Å, more preferably between about 36 Å and about 44 Å, andoptimally about 40 Å.

A second uniquely preferred low solar reflectance, low-emissivity filmstack has the following structure: (1) a zinc oxide layer depositeddirectly upon a glass sheet at a thickness of between about 108 Å andabout 148 Å, more preferably between about 115 Å and about 141 Å, andoptimally about 128 Å; (2) a titanium layer deposited directly upon thiszinc oxide layer at a thickness of between about 109 Å and about 149 Å,more preferably between about 116 Å and about 142 Å, and optimally about129 Å, wherein an outer portion of this titanium layer is oxidizedduring deposition of the overlying zinc oxide film in an oxidizingatmosphere; (3) a zinc oxide layer deposited directly upon this titaniumlayer at a thickness of between about 300 Å and about 406 Å, morepreferably between about 318 Å and about 388 Å, and optimally about 353Å; (4) a silver layer deposited directly upon this zinc oxide layer at athickness of between about 183 Å and about 247 Å, more preferablybetween about 193 Å and about 237 Å, and optimally about 215 Å; (6) atitanium layer deposited directly upon this silver layer at a thicknessof between about 53 Å and about 71 Å, more preferably between about 56 Åand about 68 Å, and optimally about 62 Å, wherein an outer portion ofthis titanium layer is oxidized during deposition of the overlying zincoxide film in an oxidizing atmosphere; (7) a zinc oxide layer depositeddirectly upon this titanium layer at a thickness of between about 200 Åand about 270 Å, more preferably between about 211 Å and about 259 Å,and optimally about 235 Å; and (8) a silicon nitride layer depositeddirectly upon this zinc oxide layer at a thickness of between about 15 Åand 21 Å, more preferably between about 16 Å and about 20 Å, andoptimally about 18 Å.

A third uniquely preferred low solar reflectance, low-emissivity filmstack has the following structure: (1) a zinc oxide layer depositeddirectly upon a glass sheet at a thickness of between about 116 Å andabout 156 Å, more preferably between about 122 Å and about 150 Å, andoptimally about 136 Å; (2) a titanium layer deposited directly upon thiszinc oxide layer at a thickness of between about 105 Å and about 141 Å,more preferably between about 111 Å and about 135 Å, and perhapsoptimally about 123 Å, wherein an outer portion of this titanium layeris oxidized during deposition of the overlying zinc oxide film in anoxidizing atmosphere; (3) a zinc oxide layer deposited directly uponthis titanium layer at a thickness of between about 321 Å and about 435Å, more preferably between about 340 Å and about 416 Å, and optimallyabout 378 Å; (4) a silver layer deposited directly upon this zinc oxidelayer at a thickness of between about 202 Å and about 274 Å, morepreferably between about 214 Å and about 262 Å, and optimally about 238Å; (5) a titanium layer deposited directly upon this silver layer at athickness of between about 58 Å and about 78 Å, more preferably betweenabout 61 Å and about 75 Å, and optimally about 68 Å, wherein an outerportion of this titanium layer is oxidized during deposition of theoverlying zinc oxide film in an oxidizing atmosphere; (6) a zinc oxidelayer deposited directly upon this titanium layer at a thickness ofbetween 177 Å and about 241 Å, more preferably between about 188 Å andabout 230 Å, and optimally about 209 Å; and (7) a silicon nitride layerdeposited directly upon this zinc oxide layer at a thickness of betweenabout 28 Å and 38 Å, more preferably between about 30 Å and about 36 Å,and optimally about 33 Å.

The present low solar reflectance coatings 40 can be applied by avariety of well known coating techniques. For example, these coatingscan be applied by sputter deposition (i.e., sputtering). Sputtering iswell known in the present art. FIG. 3 depicts an exemplary magnetronsputtering chamber 200. Magnetron sputtering chambers and relatedequipment are commercially available from a variety of sources (e.g.,Leybold and BOC Coating Technology). Useful magnetron sputteringtechniques and equipment are described in U.S. Pat. No. 4,166,018,issued to Chapin, the entire contents of which are incorporated hereinby reference.

In favored methods of the invention, the substrate 10 is coated in amultiple-chamber sputtering line. Sputtering lines are well known in thepresent art. A typical sputtering line includes a series of sputteringchambers that are aligned and connected such that a sheet-like substrate10 can be passed from one chamber to the next by conveying the substrate10 horizontally over spaced-apart transport rollers 210 in each of thechambers. Thus, the rollers 210 form a continuous path of substrate 10travel through the sputtering line. The substrate 10 is typicallyconveyed at speeds of between about 100–500 inches per minute.

In one particular deposition method, the substrate 10 is positioned atthe inlet of the sputtering line and conveyed into a first coat zone.The first coat zone is provided with three cathodes adapted to depositthe transparent dielectric inner coat 30. All three of these cathodescomprise zinc sputtering targets. The zinc targets 240 in the first coatzone are sputtered in an oxidizing atmosphere to deposit a zinc oxideinner coat 30. This oxidizing atmosphere may consist essentially ofoxygen (e.g., about 100% O₂). Alternatively, this atmosphere maycomprise Ar/O₂ (e.g., at about 3.5 mbar). A power of about 37–39 kW isapplied to each of the zinc targets, while the substrate 10 is conveyedbeneath all three of these targets at a rate of about 475 inches perminute, such that a zinc oxide inner coat 30 is applied at a thicknessof about 128 Å.

The substrate 10 is then conveyed into a second coat zone where the highabsorption primary layer 80 is applied directly over the inner coat 30.This second coat zone preferably contains an inert atmosphere (e.g.,argon at about 4 mbar). One of the sputtering bays in this coat zone hasa titanium target. A power of about 68–69 kW is applied to this titaniumtarget, while the substrate is conveyed beneath this target at a rate ofabout 475 inches per minute, to deposit a titanium high absorptionprimary layer 80 at a thickness of about 129 Å. The substrate 10 is thenconveyed through three subsequent active coat zones to deposit thetransparent dielectric middle coat 90, as will now be described.

The thus coated substrate is conveyed through a third coat zone havingthree sputtering bays each with a zinc target and then through a fourthcoat zone also having three sputtering bays each with a zinc target. Allsix of these zinc targets are sputtered in an oxidizing atmosphere (asdescribed above) to deposit the innermost portion of the middle coat 90.The substrate 10 is conveyed beneath these six targets at a rate ofabout 475 inches per minute, while a power of about 42–47 kW is appliedto each target.

The substrate 10 is then conveyed through a subsequent coat zonecontaining an oxidizing atmosphere. Two of the sputtering bays in thiszone are active and have zinc targets. The substrate is conveyed beneaththese targets at a rate of 475 inches per minute, while a power of about8–9 kW is applied to the first target and a power of about 46 kW isapplied to the second target. When the substrate 10 is conveyed beneaththese two zinc targets and the previous six zinc targets, a total ofabout 353 Å of zinc oxide is deposited directly on the titanium highabsorption primary layer 80. During deposition of this zinc oxide, theoutermost portion of the underlying titanium layer 80 is oxidized, asdescribed above.

The substrate 10 is then conveyed into a further coat zone wherein theinfrared-reflective film 150 and the high absorption blocker layer 180are deposited. This coat zone preferably contains an inert atmosphere(described above). The first two sputtering bays of this coat zone eachhave a silver target. A power of about 13–14 kW is applied to the firstsilver target and a power of about 7–8 kW is applied to the secondsilver target. The substrate 10 is conveyed beneath these two targets atabout 475 inches per minute, such that a silver infrared-reflectivelayer 150 is deposited at a thickness of about 215 Å. The thirdsputtering bay of this coat zone has a titanium target. A power of about33 kW is applied to this titanium target, while the substrate 10 isconveyed beneath this target at a rate of about 475 inches per minute,to deposit the high absorption blocker layer 180 at a thickness of about62 Å. The thus coated substrate is then conveyed through four moreactive coat zones, wherein the outer coat 120 is applied, as will now bedescribed.

The substrate 10 is conveyed through a subsequent coat zone thatincludes three sputtering bays each having one zinc target, then througha further coat zone having only one active sputtering bay with a zinctarget, and then through yet another coat zone having three activesputtering bays each with one zinc target. Each of these coat zonescontains an oxidizing atmosphere. A power of about 33–38 kW is appliedto each of the first three zinc targets, a power of about 5 kW isapplied to the fourth zinc target, a power of about 31 kW is applied tothe fifth zinc target, a power of about 37–38 kW is applied to the sixthzinc target, and a power of about 6 kW is applied to the seventh zinctarget. The substrate 10 is conveyed beneath these targets at a rate ofabout 475 inches per minute, while sputtering each target at thedescribed power level, to deposit about 235 Å of zinc oxide directlyover the high absorption blocker layer 180.

The thus coated substrate is then conveyed into a final coat zonewherein the outermost portion of the transparent dielectric outer coat120 is deposited. This coat zone has two active sputtering bays eachwith a silicon target (doped with aluminum). A nitriding atmosphere ispreferably maintained in this coat zone during sputtering. For example,this atmosphere can be nitrogen at a pressure of about 3.5–5 mbar. Apower of about 3–4 kW is applied to the first silicon target, while apower of about 25–26 kW is applied to the second silicon target. Thesubstrate 10 is conveyed beneath these targets at a rate of about 475inches per minute, while sputtering each target at the described powerlevel, to deposit about 18 Å of silicon nitride directly over theunderlying zinc oxide. This completes the low solar reflectance coating40 of one particular embodiment.

While preferred embodiments of the present invention have beendescribed, it should be understood that numerous changes, adaptations,and modifications can be made therein without departing from the spiritof the invention and the scope of the appended claims.

1. A pane bearing a low-emissivity coating comprising, in sequenceoutwardly from the pane, an inner coat, a high absorption primary layer,a middle coat, an infrared-reflective layer, and an outer coat, theinner coat comprising at least one transparent dielectric film and beingapplied directly on the pane and having an optical thickness of lessthan about 272Å, the middle coat comprising at least one transparentdielectric film and having an optical thickness of between about 600 Åand about 872 Å, said infrared-reflective layer comprising material thatis highly reflective of infrared radiation and having a thickness of atleast about 175 Å, the high absorption primary layer comprising materialthat is highly absorptive of solar radiation and having a thickness ofat least about 100 Å, wherein a major portion of the thickness of thehigh absorption primary layer is metallic titanium.
 2. The pane of claim1 wherein said infrared-reflective layer comprises silver.
 3. The paneof claim 1 wherein the low-emissivity coating includes a high absorptionblocker layer directly over said infrared-reflective layer, the highabsorption blocker layer having a thickness of between about 46 Å andabout 78 Å.
 4. The pane of claim 1 wherein the high absorption primarylayer has a thickness of between about 110 Å and about 144 Å.
 5. Thepane of claim 1 wherein each film of the middle coat has a refractiveindex of between about 1.7 and about 2.4.
 6. The pane of claim 1 furthercomprising a high absorption blocker layer deposited over saidinfrared-reflective layer, the high absorption blocker layer comprisingmaterial that is highly absorptive of solar radiation and having athickness of at least about 45 Å, wherein the high absorption blockerlayer comprises titanium.
 7. The pane of claim 6 wherein the highabsorption blocker layer is deposited directly over saidinfrared-reflective layer.
 8. The pane of claim 6 wherein a majorportion of the thickness of the high absorption blocker layer ismetallic titanium.
 9. The pane of claim 6 wherein the high absorptionblocker layer comprises an outer portion that is titanium oxide.
 10. Thepane of claim 6 wherein the thickness of the high absorption blockerlayer is between about 54 Å and about 68 Å.
 11. The pane of claim 1wherein each film of the inner coat has a refractive index of betweenabout 1.7 and about 2.4.
 12. The pane of claim 1 wherein the outer coatcomprises a layer of zinc oxide and a layer of silicon nitride over saidlayer of zinc oxide.
 13. The pane of claim 12 wherein each film in theouter coat has a refractive index of between about 1.7 and about 2.4.14. The pane of claim 1 wherein the pane is part of an insulating glassunit and the low-emissivity coating is carried on a #2 surface of theinsulating glass unit.
 15. The pane of claim 14 wherein the insulatingglass unit has an exterior solar reflectance of less than about 30%. 16.The pane of claim 15 wherein the exterior solar reflectance is less thanabout 20%.
 17. The pane of claim 16 wherein the exterior solarreflectance is about 16%.
 18. The pane of claim 1 wherein thelow-emissivity coating has an emissivity of less than about 0.07. 19.The pane of claim 18 wherein the emissivity is less than about 0.05. 20.The pane of claim 19 wherein the emissivity is about 0.044.
 21. The paneof claim 14 wherein the insulating glass unit has an exterior visiblereflectance of less than about 20%.
 22. The pane of claim 21 wherein theexterior visible reflectance is less than about 15%.
 23. The pane ofclaim 22 wherein the exterior visible reflectance is about 11%.
 24. Thepane of claim 14 wherein the insulating glass unit has a U Value of lessthan about 0.4.
 25. The pane of claim 24 wherein the U Value is lessthan about 0.3.
 26. The pane of claim 14 wherein the insulating glassunit has a solar heat gain coefficient of less than about 0.4.
 27. Thepane of claim 14 wherein the insulating glass unit has a transmittedcolor characterized by an a_(h) color coordinate of between about −1.75and about −4.5 and a b_(h) color coordinate of between about −2 andabout −5.
 28. The pane of claim 14 wherein the insulating glass unit hasan exterior reflected color characterized by an a_(h) color coordinateof between about 1.4 and about −1.6 and a b_(h) color coordinate ofbetween about 0.5 and about −2.5.
 29. A pane bearing a low-emissivitycoating comprising the following sequence of films: a) an inner, applieddirectly to the pane, coat comprising at least one transparentdielectric film and having an optical thickness of less than about 272Å;b) a high absorption primary layer comprising material that is highlyabsorptive of solar radiation and having a thickness of least about 100Å, wherein a major portion of the thickness of the high absorptionprimary layer is metallic titanium; c) a middle coat comprising at leastone transparent dielectric film and having an optical thickness ofbetween about 600 Å and about 872 Å; d) an infrared-reflective layercomprising material that is highly reflective of infrared radiation andhaving a thickness of at least about 175 Å; e) a high absorption blockerlayer comprising material that is highly absorptive of solar radiationand having a thickness of at least about 45 Å; and f) an outer coatcomprising a layer of zinc oxide and a layer of silicon nitride oversaid layer of zinc oxide.
 30. A pane bearing a low-emissivity coatingcomprising an infrared-reflective layer, a high absorption primarylayer, an inner coat, and a middle coat, the inner coat comprising atleast one transparent dielectric film and being deposited directly onthe pane, wherein each film of the inner coat has a refractive index ofbetween about 1.7 and about 2.4, said infrared-reflective layercomprising silver and having a thickness of at least about 175 Å, thehigh absorption primary layer comprising material that is highlyabsorptive of solar radiation and having a thickness of at least about100 Å, wherein a major portion of the thickness of the high absorptionprimary layer is metallic titanium, the middle coat comprising at leastone transparent dielectric film and being positioned between saidinfrared-reflective layer and the high absorption primary layer, whereinthe low-emissivity coating includes a high absorption blocker layerdirectly over said infrared-reflective layer, the high absorptionblocker layer having a thickness of at least about 45 Å, wherein thepane is part of an insulating glass unit and the low-emissivity coatingis carried on a #2 surface of the insulating glass unit, the insulatingglass unit having an exterior solar reflectance of less than about 20%and having a transmitted color characterized by an a_(h) colorcoordinate and a b_(h) color coordinate, both of said color coordinatesbeing negative such that the insulating glass unit has a transmitted huewithin a range that is entirely within a blue-green range.
 31. The paneof claim 26 wherein the insulating glass unit has a solar heat gaincoefficient of less than about 0.2.
 32. The pane of claim 31 wherein theinsulating glass unit has a solar heat gain coefficient of about 0.16.33. The pane of claim 1 wherein the outer coat comprises at least onetransparent dielectric film and has an optical thickness, the coatinghaving a ratio defined as the optical thickness of the outer coatdivided by the optical thickness of the middle coat, said ratio beingbetween about 0.52 and about 0.88.
 34. The pane of claim 29 wherein theouter coat has an optical thickness, the coating has a ratio defined asthe optical thickness of the outer coat divided by the optical thicknessof the middle coat, and said ratio is between about 0.52 and about 0.88.35. The pane of claim 30 wherein the insulating glass unit has a solarheat gain coefficient of less than about 0.2.